Increased regulation of emissions from gas turbine engines has prompted the development of thermally efficient and reduced emission fuel injectors. It is known that carbon monoxide (CO) and unburned hydrocarbons (UHC) emissions can be reduced with high temperature combustion in the engine. However, high temperature combustion can result in increased production of nitrogen oxides (NOx). This problem has been addressed by injectors which are configured more thoroughly to mix fuel with air prior to combustion with a fuel-to-air ratio below the stoichiometric level. Such arrangements can provide a “lean burn” which results in lower flame temperatures than would occur with stoichiometric burning. Since the production of NOx is a strong function of temperature, a reduced flame temperature or “lean burn” results in lower levels of NOx.
Staged fuel injection is known to result in reduced engine emissions. In such arrangements, the combustion process is divided into two (or more) zones or stages. These stages are typically physically separate from each other, but close enough to permit interaction. Separation can be axial and/or radial separation. A first, pilot stage is configured to provide fuel for low power operations. In higher power conditions, the pilot continues to provide fuel to the engine and in addition a second, main stage provides the remaining fuel needed for engine operation. In this way, proper fuel-to-air ratios can be controlled for efficient combustion, reduced emissions, and good stability.
Along with staged combustion, pollutant emissions can be reduced by providing a more thoroughly mixed fuel-air mixture prior to combustion wherein the fuel-to-air ratio is below the stoichiometric level so that the combustion occurs at lean conditions. Lean burning results in lower flame temperatures than would occur with stoichiometric burning. Since the production of NOx is a strong function of temperature, a reduced flame temperature results in lower levels of NOx. The technology of directly injecting liquid fuel into the combustion chamber and enabling rapid mixing with air at lean fuel-to-air ratios is called lean direct injection (LDI). A gas turbine engine typically comprises multiple injectors and non-variation from one injector to another is important for predictable performance of the engine.
With an ever increasing demand for higher temperature operability, the heat load on some components of the injector becomes less tolerable and the operational life of the injector limited.
It is common practice to weld components of a fuel injector. Weld joints can shrink variably from one injector assembly to the next and can distort the surrounding metal parts due to heat input thermal gradients. For example, in the weld between a heat shield and an air swirler, minute peaks and valleys of melted and re-solidified metal can result creating stress concentrations that can start fatigue cracks unpredictably and impact on the life of the component. FIG. 1 illustrates a staged fuel injector having a prior known configuration. The injector is shown in perspective view from an upstream end (FIG. 1a) and in schematic axial cross section (FIG. 1b). The injector comprises a pilot inner air swirler 101 arranged on a centreline C-C. Immediately radially outboard of the pilot inner air swirler 101 is a pilot fuel swirler 105. A main fuel swirler 106 sits radially outboard of the pilot fuel swirler 105. A main inner air swirler 102 sits radially between the pilot fuel swirler 105 and the main fuel swirler 106. In the embodiment shown, the main fuel swirler branches to provide a pilot outer air swirler 104 which sits radially between the pilot fuel swirler 105 and a radially outer branch 102. A fuel tube 107 extends radially outwardly and is arranged to deliver fuel to the main fuel swirler 106. A separate conduit 108 extends from the fuel tube 107 to the main inner air swirler 102 to deliver fuel to the pilot fuel swirler 105. At an upstream end of the injector assembly, a heat protective casing 103 is arranged to cover upstream ends of the fuel swirlers 105, 106 and fuel tube 107 (including conduit 108). The air swirlers 101, 102 and 104 remain open so as, when in use in a gas turbine engine, to receive a flow of compressed air arriving from upstream of the casing. The air flows substantially in an axial direction in parallel with the centreline C-C.
The casing 103 has an upstream facing radially extending surface which inclines from an upstream position proximal to the fuel tube 107 to a downstream position distal to the fuel tube 107. It will be appreciated that the walls of the main inner air swirler 102 terminate at different axial positions at the upstream end. This shaping has been developed to allow access behind the fuel injector for assembly, repair and maintenance and also to keep weight down.
The various components of the fuel injector are welded at the weld zones marked W1 (pilot inner air swirler 101 to casing 103), W2 (main inner air swirler 102 to casing 103) W3 (a radially outer section of the casing 103a to a radially inner section of the casing 103b), W4 (walls 102a, 102b of main inner air swirler 102 to walls of main fuel swirler 106 and pilot fuel swirler 105). Typically, these welds join very thin (for example about 0.03 inches/0.75 mm) high strength alloys together or to thicker sections.
Welding processes may be inherently difficult to control and so it may be difficult to achieve easily repeatable results on similar components. The nature of the thin-thick material and geometry of these joints means that the welding process can add more heat to some components than others. Consequently, welds may pull the components into various distortions and interferences with each other resulting in inconsistent joints and dimensional stack ups from one injector to the next. Additionally, gaps in air swirler ducts can be made inconsistent within a single injector, causing potential unwanted air disturbances and consequent inefficiencies in the fuel burn.
The back side of the welds also may be unpredictable. It can leave inherent sharp valleys that can cause cracks to start sooner than expected due to the nature of the vibrations that may be experienced by the injector when in use in a gas turbine engine. The complicated geometry is not conducive to automated welding methods. Weld beads from part to part or even on the same part can vary in size and shape small differences cumulatively having a significant effect.
There is a desire to provide a staged fuel injector which can be manufactured by a more repeatable process and hence a more consistent and predictable behaviour in operation.